Rfid communication systems and methods, and rfid readers and systems

ABSTRACT

A method of coordinating a plurality of RFID readers includes controlling the RFID readers such that only one of the readers performs an inventory of RFID tags at a time. A system for coordinating a plurality of RFID readers is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/607,263, filed on Dec. 1, 2006, granted and due to issue on Apr. 12,2011 as U.S. Pat. No. 7,924,141, the disclosure of which is herebyincorporated herein by reference.

TECHNICAL FIELD

The technical field relates to radio frequency identification. Moreparticularly, various embodiments of the invention relate to methods andapparatus for avoiding collisions between readers of radio frequencyidentification devices.

BACKGROUND

Radio frequency identification devices (RFIDs) are known in the art.Such devices are typically used for inventory tracking. As large numbersof objects are moved in inventory, product manufacturing, andmerchandising operations, there is a continuous challenge to accuratelymonitor the location and flow of objects. Additionally, there is acontinuing goal to determine the location of objects in an inexpensiveand streamlined manner. One way to track objects is by affixing RFIDtags to objects or groups of objects, and interrogating the RFID tagswith an interrogator or reader to determine which objects are present inany particular location. RFID tags may be provided with uniqueidentification numbers or codes in order to allow a reader todistinguish between multiple different tags.

Some RFID tags use the electromagnetic field from an interrogator forpower. Typically, these devices are passive (have no power supply),which results in a small and portable package.

Another type of RFID tag is an active RFID tag, which includes its ownsource of power, such as a battery.

If an interrogator or reader has prior knowledge of the identificationnumber of a device, the reader can specify that a response is requestedonly from the device with that identification number. Sometimes, suchinformation is not available. For example, there are occasions where areader is attempting to determine which of multiple devices are withincommunication range. When the reader sends a message to a transponderdevice requesting a reply, there is a possibility that multipletransponder devices will attempt to respond simultaneously, causing acollision, and thus an erroneous message to be received by the reader.For example, if the interrogator sends out a command requesting that alldevices within a communications range identify themselves, and receivesa large number of simultaneous replies, the interrogator may not able tointerpret any of these replies. Therefore, arbitration or singulationschemes are employed to permit communications that are free ofcollisions. The term singulation refers to identifying a specificindividual tag in a multiple tag environment.

In some arbitration or singulation schemes, described in commonlyassigned U.S. Pat. Nos. 5,627,544; 5,583,850; 5,500,650; and 5,365,551,all to Snodgrass et al. and the disclosures of all of which areincorporated herein by reference, a reader sends a command causing eachdevice of a potentially large number of responding devices to select arandom number from a known range and use it as that device's arbitrationnumber. By transmitting requests for identification to various subsetsof the full range of arbitration numbers, and checking for an error-freeresponse, the interrogator quickly determines the arbitration number ofevery responder station capable of communicating at the same time.Thereafter, the interrogator is able to conduct subsequent uninterruptedcommunication with devices, one at a time, by addressing only onedevice. Various arbitration or singulation schemes are discussed incommonly assigned U.S. Pat. No. 6,275,476 to Wood, Jr.; U.S. Pat. No.6,118,789 to Wood, Jr.; U.S. Pat. No. 6,072,801 to Wood, Jr. et al.; andU.S. Pat. No. 6,061,344 to Wood, Jr., the disclosures of all of whichare incorporated herein by reference.

It is possible to have multiple readers operating in the same location.Problems can arise when multiple readers try to read the same tag at thesame time. To speed up the identification process for a large number oftags, each tag has some internal method of removing itself from theinventory, after it has been identified by a reader. If a second readerwants to inventory the same population (or even read a single tag)during a first reader's inventory session, then the possibility existsthat the second reader will not read some of the tags. This is becausethe tags that have been identified by the first reader have discontinuedresponding to any reader during the inventory as it has been flagged toremove itself from inventory.

Just as two (or more) tags responding to one reader can create anunreadable-information collision at the reader, two (or more) readersbroadcasting to one tag can create an unreadable-information collisionat the tag, along with possible RF interference.

EPCglobal is a standard setting organization that is developingstandards for electronic product codes to support the use of RFIDtechnology. One of their standards, called Class 1, Generation 2 (alsoknown as “Gen 2”) applies to passive RFID systems, and is described ontheir websites at www.epcglobalus.org or www.epcglobalinc.org. Thesestandards evolve over time, and for a particular standard, such as Gen2, there are minor variations between versions. The present version ofthe Class 1, Generation 2 standard is version 1.0.9.

The EPCglobal Class 1, Generation 2 standard has implemented a methodcalled “Sessions” to attempt to solve the problem of two, three or fourreaders reading the same population of tags in the same time period;i.e., an inventory processes overlap.

According to the specification, a reader shall support and tags shallprovide four sessions, and tags shall participate in one and only onesession during an inventory round. Two or more interrogators can usesessions to independently inventory a common tag population. Tagsassociate a separate and independent “inventoried” flag to each ofseveral readers. After singulating a tag, an interrogator may issue acommand that causes the tag to invert its inventoried flag for thatsession.

However, the inventory Sessions concept, as it is described in theEPCglobal Class 1, Generation 2 specification, has at least oneimportant shortcoming. One tag receiving power from two or more readersat the same moment will be prevented from recognizing the command fromeither reader. Although, in some cases, the multiple readers will eachtry again, there is no guarantee that they will be successful, sincethey are both continuing to retry. Further, either reader may decidethat there is no tag present, and will therefore miss a tag or tags.

Even though the EPCglobal Class 1, Generation 2 specification statesthat the inventory process is “time-interleaved,” that is notnecessarily the case because of the time independence of eachinterrogator. One reader's broadcast may step on or interfere withanother reader's broadcast, causing indeterminate results. Tags facedwith trying to read both readers will not be able to interpret themingled transmissions, and may not respond at all, without regard towhich session the readers want. There is no required time coordinationbetween the two readers in the sessions concept of EPCglobal Class 1,Generation 2.

If two or more readers are performing an inventory, this can and oftenwill lead to indeterminacy in the inventory process, either in readingall the tags if the inventory process is stopped too soon, or in thelength of time it takes to get a high probability that all readers haveread all the tags within their ranges. Users expect not just a highprobability, but a 100% probability that all readers have read all thetags. The EPCglobal inventory process has so many rapid queries that theprobability that another reader (in any of the other three sessions) cancause reader interference at a tag is extremely high. The queries arerapid in order to achieve a 1000 tags/second inventory rate stated inthe specification. Reader interference includes multiple overlappingreader broadcasts. The interference can be at different tags atdifferent times.

Because the EPCglobal Sessions concept allows for four sessions, theproblem is magnified further if three and four readers are broadcastingsimultaneously. Also, the persistence times add to the indeterminancy(and therefore, unreliability) of the multiple inventory sessionsprocess.

The reader collision problem is exacerbated by the fact that each tagbeing inventoried must sometimes establish communications not once, buttwice or more with a given reader (and session). More particularly, theEPCglobal specification recites that “ . . . After singulating a Tag anInterrogator may issue a [second] command that causes the Tag to invertits inventoried flag for that session . . . ”. Notice that a tag mustfirst be singulated, which can be problematic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system according to various embodimentsof the present disclosure.

FIG. 2 is a block diagram of the reader controller and of a readerincluded in the system of FIG. 1 in one embodiment. Other embodimentsare also contemplated.

FIG. 3 is a flowchart illustrating a process executed by the readercontroller of FIG. 2, in one embodiment. Other embodiments are alsocontemplated.

FIG. 4 is a flowchart illustrating a process executed by a readercontroller, in one embodiment. Other embodiments are also contemplated.

FIG. 5 is a flowchart illustrating a process executed by multiplereaders where an inventory session is interrupted and continued later,in some embodiments. Other embodiments are also contemplated.

FIG. 6 is a flowchart illustrating a process executed by multiplereaders to determine if tags are appearing or disappearing. Otherembodiments are possible.

FIG. 7 is a portion of a table illustrating an example of implementationof the embodiment of FIG. 5.

FIG. 8 is a portion of a table illustrating an example of implementationof the embodiment of FIG. 6.

FIG. 9 is a portion of a table illustrating an example of implementationof the embodiment of FIG. 6.

FIG. 10 is a block diagram illustrating a system described in connectionwith the example of FIG. 7.

FIG. 11 is a block diagram illustrating a system described in connectionwith the example of FIG. 8.

FIG. 12 is a block diagram illustrating a system described in connectionwith the example of FIG. 9.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Various embodiments of the invention provide a method of coordinating aplurality of RFID readers, including controlling the RFID readers suchthat one and only one of the readers performs an inventory of RFID tagsat a time.

Various embodiments provide solutions to this problem with the EPCglobalClass 1, Generation 2 specification. Various embodiments provide asolution to the problem of collisions between readers regardless of whatspecification or standard is used.

FIG. 1 shows a system 10 including a plurality of RFID readers 12-17.The readers 12-17 include some handheld readers 12-14 and some fixedreaders 15-17. The system 10 further includes a population 19 of RFIDtags 30-45. Any of the RFID readers 12-17 can communicate with any ofthe RFID tags 30-45. In the embodiment of FIG. 1, only one readerbroadcasts at a time or per period of time. In some embodiments, thetags and readers otherwise follow the Class 1, Generation 2specification. In these embodiments, the inventory sessions concept willwork more reliably. This embodiment allows any number N of readers toinventory a population 19 of tags, instead of just two. The benefits ofthis embodiment are increased reliability and/or increased speed.

The reader controller 18 coordinates multiple readers 12-17. In someembodiments, the reader controller 18 only allows one reader tobroadcast at any given instant or period of time, whether or notdifferent readers are inventorying using different sessions. This isreferred to as reader multiplexing.

FIG. 2 is a block diagram illustrating additional construction detailsof the reader controller 18 and of a reader 12. The reader controller 18includes a processor 50, a power supply 52, wireless networking hardware54, and a wired output 56. The wireless networking hardware 54 cancomprise Wi-Fi (e.g., IEEE 802.11 or 802.20) circuitry, in someembodiments. At least one of the readers, for example, reader 12,includes a processor 57, a power supply 58, wireless networking hardware60, a transmitter 62, a receiver 64, and may include a wired input 66.The wireless networking hardware can comprise Wi-Fi circuitry, in someembodiments. The reader 12 is controlled by the controller 18 via thewireless networking hardware 60 and 54. In some embodiments, the reader12 is also capable of being coupled to the controller 18 by a wire orcable 68, though it includes the wireless networking hardware 60. Insome embodiments, the reader 12 is controlled by the controller 18 viathe wire or cable 68, and the wireless networking hardware is omitted.The reader 12 communicates with tags 30-45 using the transmitter 62 andthe receiver 64.

Both fixed and handheld (mobile) readers may be equipped with thewireless network hardware. The multiplexing periods can be constructedso as to be transparent to the user.

In some embodiments, any number of readers may be coordinated at thesystem level of control, by the controller 18, such that each readermust finish its inventory before the next reader is allowed to broadcastcommands to perform its inventory function. This allows for more thanfour sessions. Any desired number, N, of readers can be employed.

For example, FIG. 3 is a flowchart illustrating a process executed bythe processor 50 of reader controller 18, in some embodiments. In step70, a loop is started. The loop is repeated for values of I equaling 1to N, starting with I=1, and I is incremented each time the looprepeats.

In step 72, the reader controller 18 enables only reader I to inventorytags. In other words, one and only one reader is caused to inventorytags.

In step 74, a determination is made as to whether or not reader I hascompleted its inventorying of tags. If not, the inventorying continues.If reader I has completed its inventorying of tags, the processor 50proceeds to step 76, the loop increments, and the next reader is causedto inventory tags.

In other embodiments of the invention, the reader controller 18 isconfigured to determine whether or not to use the reader multiplexing byfirst comparing tag unique identification numbers or serial numbers readby each reader. In some embodiments, the reader controller 18 mayinclude a database 100 storing numbers of tags read by each reader, anddatabase manager software that looks for situations where two readersare attempting to read the same population of tags. If any tag serialnumbers were read by multiple readers, that means that the two readersare working on the same tag population. Such readers are multiplexed(e.g., FIG. 3 is followed for readers working on the same tagpopulation). The inventory(ies) may be restarted for these readers, butin a multiplexed manner.

For example, FIG. 4 is a flowchart illustrating a process executed bythe processor 50 of reader controller 18, in some embodiments. In step80, tag numbers read by each reader are compared and a determination ismade whether any tag numbers were read by multiple readers. If so, theprocessor 50 proceeds to step 82. If not, the processor 50 proceeds tostep 90.

In step 82, a loop is started. The loop is repeated for each of thosereaders identified in step 80 as having read tag numbers read by anotherreader.

In step 84, the reader controller 18 enables only one of those readersto inventory tags. In other words, one and only one reader is caused toinventory tags. The inventorying is restarted if the reader alreadyperformed an inventory.

In step 86, a determination is made as to whether or not reader that isinventorying has completed its inventorying of tags. If not, theinventorying continues. If the reader has completed its inventorying oftags, the processor 50 proceeds to step 88, the loop increments, and thenext reader is caused to inventory tags.

In step 90, normal operation continues without multiplexing.

This embodiment handles, for example, the situation where a handheld(mobile) reader comes into the same working volume or area as a fixedreader or another handheld, where both readers need to determineaccurate tag inventories.

In some embodiments, when the multiple readers are not reading any ofthe same tag serial numbers any more (e.g., if a reader has moved out ofthe area), the multiplexing of readers can be terminated. Thus, in someembodiments, after a delay 92 or 94, the comparison 80 is performedagain.

In other embodiments, all the multiple readers 12-17 in the system maybe multiplexed all the time.

In some embodiments, a database is used to keep track of tag serialnumbers (or any form of tag IDs) that are locked out for each readerwhile it performs inventory. This allows an inventory by any reader tobe interrupted and later resumed where it left off.

FIG. 5 is a flowchart illustrating a process executed by multiplereaders 12-17, in some embodiments, to perform any number N of sessionsby any number M of readers wherein one or more sessions are interruptedduring a reader's inventory. The process of FIG. 5 allows the session tobe resumed later without the reader missing inventorying any tags.

In step 96, under the control of the controller 18, one reader 12-17(referred to in FIG. 5 as Reader 1) begins an inventory session(referred to in FIG. 5 as Session 1). An inventory session of step 96can include, for example, an inventorying session as described in theEPCglobal Class 1, Generation 2 specification. In some embodiments, theinventory session of step 96 can include an arbitration process asdescribed in the above-mentioned Snodgrass patents. In the illustratedembodiment, Reader 1 sets a flag (or stores data) on tags that areidentified or inventoried by Reader 1 (e.g., inventoried, identified, orsingulated without collision). The flag indicates that the tag has beeninventoried. In some embodiments, the flag is a lockout bit as describedin the above mentioned Snodgrass patents. In other embodiments, the flagis a session bit as described in the EPCglobal Class 1, Generation 2specification.

In step 98, identification information (e.g., tag IDs, portions of tagIDs, or other data identifying tags) for inventoried tags is stored. Insome embodiments, the identification information for inventoried tags isstored in a database 114 for Reader 1. This database 114 is stored, insome embodiments, in Reader 1 itself (see FIG. 1).

If the reader controller 18 decides to let another of the readers 12-17,referred to in FIG. 5 as Reader 2, begin an inventory process, Reader1's inventory session is interrupted in step 100. Reader 1's inventorysession is referred to as Session 1 in FIG. 5. The reader controller 18may decide to interrupt Reader 1's inventory process for any of avariety of reasons. For example, Reader 2 may be considered to be moreimportant or higher priority than Reader 1.

In step 102, Reader 1 resets flags or data. In some embodiments, all theflags or data (e.g., lockout bits) are cleared or reset for the entiretag population. In some embodiments, the flags that were set in step 96are cleared or reset.

In step 104, Reader 2 begins its inventory session freshly. Reader 2sets a flag (or stores data) on tags that are identified or inventoriedby Reader 2 (e.g., inventoried, identified, or singulated withoutcollision).

In step 106, identification information (e.g., tag IDs, portions of tagIDs, or other data identifying tags) for inventoried tags is stored. Insome embodiments, the identification information for inventoried tags isstored in a database 116 for Reader 2. This database 116 is stored, insome embodiments, in Reader 2 itself (see FIG. 1).

In step 108, the controller 18 decides that it is time for Reader 1 (orany other reader M) to resume its inventory. This can includeinterrupting Reader 2's Session 2. In some embodiments, the Session 2flagged tag data is stored in a database 118 in Reader 1, or somewhereelse. The database 118 includes data representing Reader 2's inventorythus far.

In step 110, all the flags are reset. The resetting is performed byReader 2 in some embodiments, and by Reader 1 in other embodiments.

In step 112, Reader 1 uses the tag identification information in itsSession 1 database 114 to re-flag the tags that were previouslyidentified as locked-out in Session 1, and Reader 1 continues its sortfrom where it left off.

Other readers can similarly be interrupted and later continue theirinventory sessions. Because the readers can communicate with each otheras well as with the reader controller, the various databases can bestored wherever desired.

FIG. 6 is a flowchart illustrating a process for addressing the problemof tag populations that change during an inventory session.

In step 120, the readers 12-17 are controlled so that all tags in rangeare read or inventoried by a first reader. Reader multiplexing, asdescribed above, is performed so that collisions and interference do notperturb the inventory results. This increases the likelihood that alltags in range are read by a first reader. More particularly, in someembodiments, the readers are controlled so that one and only one readerat a time performs an inventory. A list of identification information(e.g., tag IDs, portions of tag IDs, or other data identifying tags) forinventoried tags is stored; e.g., in the first reader.

In step 122, the inventorying of step 120 is repeated (e.g., by thefirst reader).

In step 124, the list of inventoried tags (e.g., tag IDs, portions oftag IDs, or other data identifying tags) from the inventorying of step120 by the first reader is compared with the list of inventoried tagsfrom the inventorying of step 122.

In step 126, the comparison of step 124 is used to determine if originaltags are disappearing or if new tags are appearing.

In step 128, appropriate action is taken depending on whether originaltags are disappearing or new tags are appearing. A variety of differentactions can be taken. This method reveals whether the inventory ischanging and how it is changing. It may be used in conjunction with themultiplexing methods described in the preceding Figures. In theillustrated embodiment, the steps of FIG. 6 are repeated, and a list oftags for each inventory session is kept.

Particular examples will now be provided to better enable one ofordinary skill in the art to understand the embodiments of FIGS. 5 and6. These are examples only; other situations are, of course, possible.

FIG. 7 illustrates an example of implementation of the embodiment ofFIG. 5. As a simplification, only three tags and two readers areillustrated. In an actual implementation, there would likely be a muchlarger population of tags and more readers. In this example, all tagsare in view of all readers, as shown in FIG. 10.

At time 210, an inventory sequence is started. All tags have lockoutbits equal to zero, and both readers illustrated in the example are off.

At time 211, the controller 18 turns on Reader 1 only.

At time 212, Reader 1 finds and locks out Tag 1.

At time 213, Reader 1 finds and locks out Tag 2

At time 214, it is time for whatever reason to switch to Reader 2.

At time 215, the controller 18 tells Reader 1 to reset all its lockedout tags in Database 1 (corresponding to database 114 in FIG. 1). Notethat all tags have lockout bits equal to zero.

At time 216, Reader 1 is turned off.

At time 217, only Reader 2 is turned on.

At time 218, Reader 2 finds and locks out Tag 3.

At time 219, it is time, for whatever reason, to switch back to Reader1.

At time 220, Reader 2 resets all its locked out tags in Database 2. Notethat now all tags have lockout bits equal to zero.

At time 221, Reader 2 is turned off.

At time 222, Reader 1 is turned on.

At time 223, from Database 1, Reader 1 sets lockout bits on Tag 1 andTag 2.

At time 224, Reader 1 finds and locks out Tag 3.

At time 225, Reader 1 cannot find any more tags.

At time 226, the controller 18 tells Reader 1 to reset all tag lockoutbits to zero.

At time 227, the inventory session for Reader 1 is finished.

At time 228, the controller 18 turns off Reader 1.

At time 229, the controller 18 decides to resume the inventory forReader 2.

At time 230, the controller 18 turns on Reader 2.

At time 231, Reader 2 sets tag lockouts per its database; i.e., Tag 3.

At time 232, Reader 2 finds Tag 2 and sets Tag 2's lockout bit.

At time 233, Reader 2 finds Tag 1 and sets its lockout bit.

At time 234, Reader 2 cannot find any more tags. Inventories arecomplete.

At time 235, the controller turns off both readers.

FIG. 8 is a portion of a table illustrating an example of implementationof the embodiment of FIG. 6. In this example, Tag 3 leaves the field ofReader 1 as shown in FIG. 11. Repeating the inventory process shown inFIG. 7 produces the inventories shown in FIG. 8. The controller 18compares the first and second inventory and finds that Tag 3 is nolonger in the field of Reader 1 but is still in the field of Reader 2.

FIG. 9 is a portion of a table illustrating an example of implementationof the embodiment of FIG. 6. In this example, Tag 4 enters the field ofonly Reader 1 as shown in FIG. 12. Repeating the inventory process shownin FIG. 7 produces the inventories shown in FIG. 9. The controller 18compares the first and second inventory and finds that Tag 4 has enteredthe field of Reader 2 but is not in the field of Reader 1.

In some embodiments, if, for any reason, a first reader 12-17 (seeFIG. 1) is deselected or turned off, a second reader 12-17 may beactivated only if no significant reader energy is detected in thevicinity of the second reader. For example, the controller causes thesecond reader to determine if it is able to detect RF from a reader and,if not, the controller activates the second reader. If there is RFpresent, a different reader can be selected where reader RF is notpresent. Detecting RF from a reader may comprise determining if an RFabove a certain predetermined threshold, of the type expected from areader, is present. Thus, in some embodiments, controlling the readers12-17 comprises selecting readers, and if a selected reader isdeselected by the controller, another reader is selected by thecontroller such that there is no reader energy, above a predeterminedthreshold, at the new selected reader.

In compliance with the statute, the subject matter disclosed herein hasbeen described in language more or less specific as to structural andmethodical features. It is to be understood, however, that the claimsare not limited to the specific features shown and described, since themeans herein disclosed comprise example embodiments. The claims are thusto be afforded full scope as literally worded, and to be appropriatelyinterpreted in accordance with the doctrine of equivalents.

1. A method of coordinating a plurality of radio frequencyidentification device (RFID) readers, the method comprising: controllingthe RFID readers such that only one of the RFID readers performs aninventory of RFID tags, in communications range, at a time.
 2. A methodin accordance with claim 1 wherein controlling the RFID readers includescontrolling handheld RFID readers.
 3. A method in accordance with claim1 wherein the controlling comprises selecting RFID readers, and wherein,if a selected RFID reader is deselected, another RFID reader is selectedonly if there is no reader energy above a predetermined threshold,detected at the second RFID reader.
 4. A method of coordinating aplurality of RFID readers capable of using different sessions, the RFIDreaders being configured to be used with RFID tags that maintain flagsfor respective sessions that indicate whether or not the tag has beeninventoried for a session, the method comprising: controlling the RFIDreaders such that only one of the RFID readers performs an inventory ofRFID tags at a time.
 5. A method in accordance with claim 4 wherein atleast some of the RFID readers include wireless networking hardware andwherein the controlling comprises controlling using the wirelessnetworking hardware.
 6. A method in accordance with claim 4 wherein atleast some of the RFID readers have Wi-Fi communications capability, andwherein the controlling comprises controlling using the Wi-Ficommunications capability.
 7. A method in accordance with claim 4wherein at least some of the RFID readers have Wi-Fi communicationscapability, and communicate with the tags other than using Wi-Ficommunications, and wherein the controlling comprises controlling usingthe Wi-Fi communications capability.
 8. A method in accordance withclaim 4 wherein the controlling comprises selecting RFID readers, andwherein, if a selected RFID reader is deselected by the controller,another RFID reader is selected by the controller such that there is noRFID reader energy above a predetermined threshold, at the new selectedRFID reader.
 9. A method comprising: determining if multiple RFIDreaders have inventoried a common RFID tag and, if so, for those RFIDreaders, inventorying tags one RFID reader at a time.
 10. A method inaccordance with claim 9 wherein the inventorying tags one RFID reader ata time comprises restarting inventorying that may have already takenplace.
 11. A method in accordance with claim 9 wherein the inventoryingone RFID reader at a time comprises selecting RFID readers using acontroller, and wherein, if a selected RFID reader is deselected by thecontroller, another RFID reader is selected by the controller such thatthere is no RFID reader energy above a predetermined threshold, at thenew selected RFID reader.
 12. A method of coordinating a plurality ofRFID readers capable of using different sessions, the RFID readers beingconfigured to be used with RFID tags that maintain flags for respectivesessions that indicate whether or not the tag has been inventoried for asession, the method comprising: controlling the RFID readers such thatonly one of the RFID readers performs an inventory of RFID tags at atime, wherein the inventorying tags one RFID reader at a time comprisesrestarting inventorying that may have already taken place.
 13. A methodin accordance with claim 12 wherein at least some of the RFID readersinclude wireless networking hardware and wherein the controllingcomprises controlling using the wireless networking hardware.
 14. Amethod in accordance with claim 13 wherein at least some of the RFIDreaders have Wi-Fi communications capability, and wherein thecontrolling comprises controlling using the Wi-Fi communicationscapability.
 15. A method in accordance with claim 12 wherein at leastsome of the RFID readers have Wi-Fi communications capability, andcommunicate with the tags other than using Wi-Fi communications, andwherein the controlling comprises controlling using the Wi-Ficommunications capability.
 16. A method in accordance with claim 12wherein the controlling comprises selecting from the RFID readers, andwherein, if a selected RFID reader is deselected by the controller forany reason, another RFID reader is selected by the controller such thatthere is no RFID reader energy above a predetermined threshold, at thenew selected RFID reader.
 17. A method of observing changes in RFID tagpopulations, the method comprising: controlling multiple RFID readers sothat only one of the RFID readers conducts an inventory of RFID tags incommunications range; storing a first list of tags inventoried by theRFID reader; repeating the inventory; storing a second list of tagsinventoried in the repetition of the inventory; and comparing the firstlist to the second list to determine if tags are entering or leavingcommunications range of at least one of the RFID readers.
 18. A methodin accordance with claim 17 wherein additional of the multiple RFIDreaders also conduct an inventory of the RFID tags, but only one of theRFID readers conducts an inventory at a time.
 19. A method in accordancewith claim 17 wherein repeating the inventory comprises using the oneRFID reader that conducted the first mentioned conducting of aninventory.
 20. A system for coordinating a plurality of RFID readers,the system comprising: a reader controller configured to control theRFID readers such that only one of the RFID readers performs aninventory of RFID tags, in communications range, at a time.
 21. A systemin accordance with claim 20 and further comprising a plurality of RFIDreaders coupled to the reader controller, wherein the RFID readersinclude handheld RFID readers.
 22. A system in accordance with claim 20wherein the controller is configured to selecting from the RFID readers,and wherein, if a selected RFID reader is deselected by the controller,another RFID reader is selected by the controller where RFID readerenergy above a predetermined threshold is not detected at the newselected RFID reader.
 23. An RFID communications system, comprising: aplurality of RFID readers capable of using different sessions, the RFIDreaders being configured to be used with RFID tags that maintain flagsfor respective sessions that indicate whether or not the tag has beeninventoried for a session; and a controller configured to control theRFID readers such that only one of the RFID readers performs aninventory of RFID tags at a time.
 24. A system in accordance with claim23 wherein at least some of the RFID readers include wireless networkinghardware and wherein the RFID controller is configured to control theRFID readers using the wireless networking hardware.
 25. A system inaccordance with claim 23 wherein at least some of the RFID readersinclude Wi-Fi hardware, and wherein the RFID controller is configured tocontrol the RFID readers using the Wi-Fi hardware.
 26. A system inaccordance with claim 25 wherein at least some of the RFID readers haveWi-Fi communications hardware, and are configured to communicate withthe tags other than using Wi-Fi communications, and wherein the RFIDcontroller is configured to control the RFID readers using the Wi-Fihardware.
 27. A system in accordance with claim 25 wherein one of theRFID readers comprises the RFID controller.
 28. A system in accordancewith claim 23 wherein the controller is configured to selecting from theRFID readers, and wherein, if a selected RFID reader is deselected bythe controller, another RFID reader is selected by the controller whereRFID reader energy above a predetermined threshold is not read at thenew selected RFID reader
 29. A system comprising: an RFID controllerconfigured to determine if multiple RFID readers have inventoried acommon RFID tag, by communicating with the multiple RFID readers andcomparing lists of RFID tags inventories by RFID readers, and, if so,for those RFID readers, causing the RFID readers to inventory tags oneRFID reader at a time.
 30. A system in accordance with claim 29 whereinthe controller is configured to cause a RFID reader that has inventoriedthe same RFID tag as another RFID reader to restart inventorying.
 31. Asystem in accordance with claim 29 wherein the controller is configuredto select from the RFID readers, and wherein, if a selected RFID readeris deselected by the controller, another RFID reader is selected by thecontroller which does not read RFID reader energy above a predeterminedthreshold, which RFID reader energy indicates that another reader isnearby and active.
 32. An RFID communications system, comprising: aplurality of RFID readers capable of using different sessions, the RFIDreaders being configured to be used with RFID tags that maintain flagsfor respective sessions that indicate whether or not the tag has beeninventoried for a session; and a controller coupled to the RFID readersand configured to control the RFID readers such that only one of theRFID readers performs an inventory of RFID tags at a time, even thoughthe RFID readers are capable of using different sessions, wherein thecontroller is configured to selectively cause a RFID reader to restartits inventory.
 33. A system in accordance with claim 32 wherein at leastsome of the RFID readers include wireless networking hardware andwherein the controller is configured to communicate with those RFIDreaders using the wireless networking hardware.
 34. A system inaccordance with claim 32 wherein at least some of the RFID readers haveWi-Fi communications hardware and wherein the controller is configuredto communicate with those RFID readers using the Wi-Fi communicationshardware.
 35. A system in accordance with claim 32 wherein at least someof the RFID readers have Wi-Fi communications hardware, and areconfigured to communicate with RFID tags other than using Wi-Ficommunications, and wherein the controller is configured to communicatewith those RFID readers using the Wi-Fi communications hardware.
 36. Asystem in accordance with claim 32 wherein the controller is configuredto select from the RFID readers, and wherein, if a selected RFID readeris deselected by the controller, another RFID reader is selected by thecontroller which does not read RFID reader energy above a predeterminedthreshold, which RFID reader energy indicates that another reader isnearby and active.
 37. A system for observing changes in RFID tagpopulations, the system comprising: a first RFID reader; a readercontroller configured to control multiple RFID readers so that only thefirst RFID reader conducts an inventory of RFID tags in communicationsrange at a time, the reader controller causing the first RFID reader toconduct a first inventory session and the reader controller causing thefirst RFID reader to conduct a second inventory session after the firstinventory session; a database for storing a first list of tagsinventoried by the RFID reader in the first inventory session; and asecond database for storing a second list of tags inventoried by thefirst RFID reader in the second inventory session, the reader controllerbeing configured to compare the first list to the second list todetermine if tags are entering or leaving communications range of theRFID reader.
 38. A system in accordance with claim 37 and furthercomprising a second RFID reader controlled by the reader controller.